Most paging systems employ receive-only personal paging units. These paging units, called pagers, can receive pages, but cannot acknowledge whether a page has been received or transmit any other kind of information. To acknowledge page receipt, or transmit other information, the page recipient must communicate with the sender using another system, such as a telephone.
In contrast, two-way paging systems employ personal paging units that can receive pages and also acknowledge receipt of pages. Additionally, many paging system equipment providers are developing two-way paging systems that can transmit other information, such as responses to electronic mail ("E-mail") messages.
Consumers of telecommunication and paging services prefer two-way paging systems because these systems are more convenient for exchanging personal communications as compared to one-way, or receive-only paging systems. Thus, paging system equipment providers are working to produce reliable, cost-efficient two-way paging systems.
Generally, two-way paging systems include transmitters for sending pages to the personal paging units, receivers for receiving acknowledgments or other inotrtnation from the personal paging units, and a central controller that is linked to the receivers and transmitters. In some two-way paging systems, the transmitters send pages to individual personal paging units with a command that a given unit acknowledge page receipt, or send other information, during a certain time slot or period. Thus, the transmitters can command personal paging units to transmit acknowledgments or return signals during different time slots to prevent congestion. Alternatively, a user of the personal paging unit may take some action to initiate a response signal, such as pressing a button on the paging unit. Such a personal paging unit may be programmed to transmit a negative acknowledgment signal if the user has not taken action within a specified time duration.
In such systems where pagers transmit information as well as receive it, the signals transmitted to the pagers can be made very powerful because the transmitters can be powered from a commercial AC power system, can be physically large and can use large antennas mounted on towers or buildings. This enables them to transmit to pagers which have small receiving antennas and which are dispersed over a wide area. Generally the area covered by a single large transmitter is referred to as a "cell".
The pagers, however, are limited in power that they can transmit back to fixed receiving sites due to the need to keep the pagers small, to use small antennas and to use moderate power from small batteries while retaining long battery life. These circumstances require very sensitive receivers at fixed receiving sites throughout the cell. Typically a plurality of receiving sites are required in the area covered by the transmitter. With multiple receivers, more than one receiver may detect a response signal from a pager.
Prior art systems have looked for the strongest pager response signal received at an individual receiver and declared it the "correct" received signal. Nevertheless, various conditions may interfere with the reception of the response signals by the receivers. Atmospheric conditions, building structures, competing signals, low battery power conditions, electromagnetic interference, and distances between the personal paging units and the receivers are examples of these conditions. Interference may result in the reception of an incorrect signal or a signal with some level of uncertainty as to whether the signal is correct. Several methods exist to reduce the interference and therefore the uncertainty. One is to increase the strength of the response signals. Increasing the strength of the response signal, however, generally requires increasing the size of the personal paging unit or decreasing its battery life. Another solution is to increase the geographic density of receivers. Costs and permit considerations limit the number and location of receivers that can be installed.
A solution that allows receivers to be limited to the existing locations of transmitters or that minimizes the number of additional receiver sites beyond the existing number of transmitter sites is desirable.